1. Field of the Invention
The invention pertains to a release device for a friction clutch of a motor vehicle, having a diaphragm spring with spring tongues having radially inner ends which can be moved axially in opposition to a restoring force in order to release the clutch, the spring tongues pivoting so that the inner ends move radially as they move axially. The device includes a guide tube; a sliding sleeve received on the guide tube and movable axially with respect to the guide tube; a release bearing mounted coaxially to the sliding sleeve, the release bearing having a first bearing race which is fixed against rotation with respect to the sliding sleeve, a second bearing race which is rotatable with respect to the first bearing race, and a plurality of rolling elements between the bearing races; and a connecting element which is located between the second bearing race and the inner ends of the spring tongues.
2. Description of the Related Art
A release device of this type serves to actuate a vehicle clutch equipped with a release element in the form of a diaphragm spring, where the free, radially inward-directed ends of the diaphragm spring rest against a roller bearing mounted on a sliding sleeve. To perform a release movement, a release crank or actuator acts on the sliding sleeve. Thus, if the clutch is of the push type, the sliding sleeve moves axially toward the clutch, and the tongues of the diaphragm spring, which are supported on the release bearing by way of a connecting element, undergo some deformation initially and then become essentially rigid. They thus act now as levers, which pivot around a center of rotation on the clutch housing, as a result of which the load being exerted on the pressure plate of the clutch is released. The means that, in the contact area between the releasing device and the diaphragm spring, the initial axial movement of the clutch release bearing is converted into a more-or-less circular pivoting movement of the diaphragm spring. The ends of the diaphragm spring tongues are therefore shifted radially inward, which is necessarily accompanied by friction between the ends of the diaphragm spring tongues and the axial contact surface of the releasing device. It is therefore necessary to apply not only the releasing force itself but also the effective frictional force, and this is expressed as a hysteresis in the characteristic force curve of the clutch pedal and leads as a consequence to an increase in the pedal force. The longer the vehicle is in service, the greater the wear at the contact point, which leads to an even greater increase in the frictional losses.
To improve the clutch-release properties, it is known from DE 101 36 424 C1, for example, that a plastic disk can be installed axially between the tongues of the diaphragm spring and the co-rotating ring of the clutch release bearing, as a result of which the frictional losses can be reduced. Systems of this type are subject to wear, however, and become ineffective after the releasing device has been in operation for a certain period of time.